The present invention relates to a protective paste for the manufacturing of electronic modules and more particularly for the manufacturing of Hybrid Multi Chip Modules.
The hybrid organic laminate chip package or Hybrid Multi Chip Module (HMCM) is more and more used in electronics due to its small dimensions and high performance. A hybrid package, as shown in FIG. 1 has a substrate 101 on which at least one Surface Mount Technology (SMT) component 103 and at least one wire bonded chip 105 are mounted together.
SMT is a well known technique for electronic card assembly. SMT usually consists of the following main steps:
Screening of solder paste on the pads of the Printed Circuit Board (PCB). The paste screening is done by means of a stencil which is laid on the substrate and a squeegee (usual a rubber blade) which spread the solder paste on the required spots;
Placement of components on the paste. This operation is performed with dedicated machines, having different speeds and precision according to the component dimension and weight.
Reflow of the solder paste under air or nitrogen, depending on the paste flux. Very active fluxes, that protect the powder alloy from the oxidation, allow an air atmosphere to be used. Normally, this reflow operation is performed at high temperature in the range 200xc2x0 C.-240xc2x0 C.
The usual wire bonded chip assembly process comprises substantially the following steps:
Die attaching of the chip to the substrate with thermal adhesive dispensing and curing (i.e. polymerisation). The curing process is usually performed at 150xc2x0 C. for 3-5 hours.
First and second bonding of the two wire ends respectively to the aluminium pad on the chip (also known as ball bonding operation) and to the gold pad on the substrate (also known as wedge bonding operation). This operation comprises connecting the wire and the pad with the application of a compression force and ultrasonic energy by a vibrating probe.
The bonding operations cause the mutual diffusion of the two metals into each other (gold-gold and gold-aluminium). This diffusion is due to the movement of the surface layer atoms, and it depends strongly on the cleanliness of the surface. In fact the surface contamination of the gold and aluminium pads (i.e. the presence of atoms not belonging to the metal lattice) behaves like a barrier against the atom movement, so decreasing or totally preventing the wire bonding. For this reason the metal pads need to be completely clean for good bonding with the wires.
In a HMCM, where both SMT and wire bonded chip attach processes must be performed, it is usual to do the wire bonded chip attach first, followed by the SMT. As shown in FIG. 2, the presence of the wire bonded chip 105 on the substrate 101 when the SMT process starts, prevents the solder paste being dispensed by screening and an alternative paste dispensing method is needed. This is because the stencil cannot be properly laid on the substrate and the squeegee cannot run on the stencil to dispense the paste on the pads 201. A known alternative dispensing method is the point by point dispensing by means of a syringe, which is a much more expensive process than the screening method. On the other hand the inversion of SMT and wire bonded chip attachment processes would cause problems due to the contamination of the gold pads during the SMT process. This contamination is mainly due to the following factors:
solder paste spreading during the solder paste screening;
solder splattering from SMT components during the solder paste reflow;
organic and tin/lead vapours produced during the solder paste reflow and depositing on the substrate.
Solder splattering, in particular, is a thick and very dangerous contamination, that compromises the gold pads bondability. This unwanted contamination is very difficult to be removed and needs very expensive treatment, e.g. plasma cleaning. Plasma cleaning is a powerful technique applied in several industrial fields, such as mechanics, electronics, optics. Plasma, also called xe2x80x98fourth state of matterxe2x80x99, is produced by applying an energy field to a gas, causing its ionization. The electron and the ions, accelerated by the energetic field, achieve a kinetic energy that can be transferred to the surface of an object that has to be cleaned and, if it is higher than the cohesion force of the surface atoms of the object, the plasma is able to remove them. However it could happen that for cleaning off a hard organic film (like flux residues of the solder paste) it needs to achieve a plasma kinetic energy so high that it overcomes the sputtering threshold of the other materials present on HMCM, so that they are damaged.
It is an object of the present invention to provide a technique which alleviates the above drawbacks.
According to the present invention, we provide a protective paste for protecting metal circuits during the manufacture of electronic modules comprising a citrate salt dissolved in water and glycerol, the weight ratio between citrate and glycerol being between 0.1 and 2.
Furthermore, according to the present invention we provide a method for manufacturing a multi chip module having on the same substrate at least one wire bonded chip and at least one Surface Mount Technology (SMT) chip, the method comprising the steps of:
protecting the gold circuits and pads to which the wire bonded chip will be connected by dispensing a layer of a protective paste, screening the paste on the circuits by means of a stencil, the paste comprising a citrate salt dissolved in water, and glycerol, the weight ratio between citrate and glycerol being between 0.1 and 2:
drying the layer of dispensed paste;
mounting the SMT chip;
removing the protective layer from the gold circuits and pads;
attaching and connecting the wire bonded chip.